Imperiled Funding Threatens Long-Baseline Neutrino Experiment

By Michael Lucibella

Physicists are fighting hard to save the country’s proposed flagship neutrino experiment from a potential death of a thousand cuts. High energy physicists and the Department of Energy have deemed the Long Baseline Neutrino Experiment (LBNE) a top priority for the US science program, but looming budget cuts at the agency have prompted numerous reviews and reductions in scope for the project.

At the August High Energy Physics Advisory Panel meeting in Rockville, MD, advisors to the DOE presented three possibilities for the first phase of the proposed neutrino detector. They warned that the scope of the research could be severely undercut by inadequate funding. Scientists ideally want to locate the detector underground and 1300 miles away from the source of the neutrinos, but the budget may not be available to do both.

“It’s a broad and very rich program of science that we want to do,” said Milind Diwan a physicist at Brookhaven National Lab and spokesperson for LBNE. “We want to build a capable large detector and locate it deep.”

The LBNE would make use of a new class of liquid argon neutrino detector located several hundred kilometers from a powerful neutrino beam originating at Fermilab. It would try to find the mass hierarchy in neutrinos, as well as look for evidence of charge parity (CP) violation. If the detectors are located underground, LBNE could also look for evidence of proton decay and neutrinos from a supernova.

The experiment has had a history of financial trouble. Initially, it was conceived as part of a large underground laboratory called DUSEL in the Homestake mine in South Dakota. In December of 2010, the National Science Foundation backed out of the project, and the research was pared to three major underground experiments at the mine, including a dark matter detector and one to look for neutrinoless double beta decay. The Department of Energy balked at the almost $2 billion price tag, and asked the committee for a cheaper option.

On August 6, the LBNE Reconfiguration Steering Committee issued its report, which recommended building a 10 kiloton argon surface detector in South Dakota, and a new beamline at Fermilab. The committee had been charged with evaluating different locations and configurations for the experiment.

The estimated cost for the committee’s preferred option is about $789 million, with an additional $135 million for locating the detector underground. The other two major options, including a 30-kiloton surface detector at Ash River in Minnesota, and a 15-kiloton underground detector at the Soudan Mine in Minnesota, are both about $100 million cheaper, but have a more limited scientific scope.

“$700 million doesn’t give you much physics at all [at Homestake],” said Young-Kee Kim, a physicist at the University of Chicago, and chair of the report committee. She noted that “a longer baseline allows complete separation between matter and CP effects.”

Scientists are pushing hard for the Homestake site because it would produce the clearest final results of the three sites. Neutrinos change types as they fly at near the speed of light, a process called oscillation. Recent results from China’s Daya Bay neutrino detectors indicate that Homestake’s location would put it at an ideal spot for observing the most dramatic changes in neutrino composition. The 1300 kilometers between Fermilab and LBNE gives neutrinos enough time and distance to substantially change how many of each kind there are in the beam. The other two possible sites would still be able to detect some neutrino change, but at a lower confidence level.

Homestake would also be able to better differentiate differences in oscillation between neutrinos and anti-neutrinos. Scientists hope to use this information to look for evidence of CP violation, which would help explain why matter came to dominate antimatter in the early universe.

The DOE’s final decision will come sometime in the fall. The representatives at the HEPAP meeting from the administration were non-committal about which plan they thought had the best chance of being funded.

“I see this LBNE thing as something we really want to do, but if we can pull it off is another story,” said Bill Brinkman, head of the DOE Office of Science. He added that top priorities in the Department of Energy have been related to energy conservation and combating climate change. “When we, myself and [Secretary] Steve Chu and other management, start to think about budgets, we tend to start to think about this first.”

Jim Siegrist, head of the DOE Office of High Energy Physics, said that in recent years, there was an unusually strong emphasis on research, and that was likely to change.

“We have under-invested in new facilities in the recent past. Correcting this will squeeze research for several years,” Siegrist said.

Fermilab has been counting on the project to be its new flagship experiment. After the Tevatron was shuttered last year, the lab had hoped to upgrade the accelerator’s old main injector for LBNE, using it to fire the 1300 kilometer beam of neutrinos at the distant detectors. Early plans had a new “near detector” at Fermilab to measure the beam at its source; however, this is now unlikely to be built until later in the project. The lab is in the process of gearing up to refocus itself more towards neutrino research.

“Neutrinos have surprised us in the past and I believe they will continue to surprise us,” said Stephen Parke, a physicist at Fermilab.

Members of the steering committee said they were in talks with foreign nations that might be interested in contributing funds to the program. Kim said that they had been talking with India, Italy and the United Kingdom, but it was likely that the European nations would want to wait until the European Strategy for Particle Physics for the next several years is agreed to before deciding to contribute to any project. Kim said that they are hoping to hear from India’s government before the end of the year.